Eggshell ultrastructure, properties and the process of mineralization : involvement of organic matrix in the eggshell fabric

Abstract

The eggshell of the hen is a highly ordered and mineralized structure, which is sequentially deposited within an acellular milieu - the uterine fluid secreted by the distal oviduct. Each day, the hen exports a huge amount of calcium to form the eggshell and undergoes tremendous changes in its calcium metabolism. The eggshell results from spherulitic crystal growth of calcite initiated on organic aggregates on the surface of eggshell membranes, followed by competition between radial crystallites for space, resulting in a compact columnar biomineral. The exceptional mechanical properties associated with the well-defined eggshell ultrastructure and its texture arise from the control of crystal morphology and growth by the organic matrix, amongst them proteins specific to the uterus and eggshell (ovocleidins and ovocalyxins). The uterine fluid constituents varies at different stages of egg calcification, and alter the morphology of calcite crystals grown in vitro. Moreover, there is a relationship between levels of matrix proteins or their single nucleotide polymorphisms upon eggshell texture or mechanical properties. All these features point to the control of eggshell fabric by the organic matrix